Clocks

AN2586

12/28

Doc ID 13675 Rev 7

2.1.1

External source (HSE bypass)

In this mode, an external clock source must be provided. It can have a frequency of up to:

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24 MHz for STM32F100xx value line devices

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25 MHz for STM32F101xx, STM32F102xx and STM32F103xx devices

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50 MHz for connectivity line devices

The external clock signal (square, sine or triangle) with a duty cycle of about 50%, has to

drive the OSC_IN pin while the OSC_OUT pin must be left in the high impedance state (see

Figure 7 and Figure 6).

2.1.2

External crystal/ceramic resonator (HSE crystal)

The external oscillator frequency ranges from:

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4 to 16 MHz on STM32F101xx, STM32F102xx and STM32F103xx devices

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4 to 24 MHz for STM32F100xx value line devices

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3 to 25 MHz on connectivity line devices

The external oscillator has the advantage of producing a very accurate rate on the main

clock. The associated hardware configuration is shown in Figure 7.

The resonator and the load capacitors have to be connected as close as possible to the

oscillator pins in order to minimize output distortion and startup stabilization time. The load

capacitance values must be adjusted according to the selected oscillator.

25 pF range (typ.), designed for high-frequency applications and selected to meet the

crystal manufacturer typically specifies a load capacitance that is the series combination of

Refer to the electrical characteristics sections in the datasheet of your product for more

details.